Three series of experiments on the stimulus properties of drugs will be performed: The first project will develop methods for drug categorization based on drug discrimination procedures. Several new training procedures will be used in this project, and we will measure the specificity of drug classification achieved by each method. It is planned to develop methods that can split the members of a pharmacological class into several subgroups, are specify the relationships of new drugs to preexisting members of the class. The second project will localize the brain mechanisms responsible for state dependent learning (SDL). Rats will be trained to discriminate the presence of drugs injected into individual brain structures in order to identify the sites responsible for the drugs' discriminable effects. Identification of the brain structures were locally applied drugs produce SDL might identify the mechanisms that mediate SDL. The third project will investigate whether drugs can act as conditioned stimuli (CSs), and acquire conditioned responses (CRs) that they did not initially produce. We will conduct experiments to determine what types of conditioning procedures can attach new CRs to drug, how prolonged the conditioning procedures have to be, and how permanent or fragile are the CRs so established. The results will indicate the likelihood that such CRs might significantly alter the efficacy of drugs during clinical use.